How to access user installed menubar icons with the keyboard?

I understand that Ctrl + F8 can be used to access the built-in Apple menu and Ctrl + F2 can be used to access program menus (e.g. this existing answer).

However, I have a bunch of user-installed items in the menu bar (e.g. Better Touch Tool, Caffeine, Alfred, Dropbox, etc.) and neither of the above key combinations allow access to these items.

How can I access the user-installed items in the menu bar using the keyboard?

Install Bartender. I'm a huge fan for many reasons; but amongst those, it allows Spotlight-like quick-filtering of menubar items:

⇧⌘␣ …

… drop …

↩!

Look, ma! No mouse. 🙌